1. Field of the Invention
The present invention relates to a rotation detecting, apparatus that detects a rotational state of a direct-current motor, such as a rotation angle or a rotational direction, and relates to a rotation detecting system having the rotation detecting apparatus.
2. Description of Related Art
A direct-current brush motor (hereinafter referred to as a “direct-current motor”) has been conventionally employed in a vehicle. For example, the direct-current motor adjusts opening/closing angles of an air mix damper for adjusting temperature and of a mode damper for switching air outlets of an air conditioner of the vehicle. In the control of the above direct-current motor, a rotational state of the direct-current motor, such as a rotation angle, a rotational direction, a rotational speed, is detected in order to accurately adjust the opening/closing angle of each damper. Then, the above direct-current motor is controlled based on the detected rotational state such that the opening/closing angle of each damper coincides with the desired angle.
A general method for detecting the rotational state of the direct-current motor includes a well-known method, in which a sensor, such as a rotary encoder, a potentiometer, is used, and the rotational state is detected based on the detection signal from the sensor. Thus, the method of detecting the rotational state by using the sensor is employed in the vehicle.
However, in the above method of detecting the rotational state by using the sensor, a sensor-mount space is required for each direct-current motor. Furthermore; additional harnesses for transmitting detection signals of the sensors to another apparatus, such as an on-board ECU, are required in addition to a harness provided to each direct-current motor for supplying direct-current power to the direct-current motor therethrough. As a result, the above requirement increases the weight of the vehicle and increases a cost of manufacture. Thus, sensorless type has been required in order to reduce the numbers of the sensor and the related harnesses.
There have been proposed various sensorless methods for detecting for the rotational state of the direct-current motor without using a large-scale or an extensive sensor, such as the rotary encoder. For example, there is known a method for detecting and counting a surge pulse generated when the connection state between the commutator and the brushes is switched. However, in the above method, the erroneous detection may be more likely to occur when the rotational speed is further reduced because the surge pulse becomes more difficult to detect when the rotational speed is reduced substantially. The above is true because during the operation of the motor under the low speed rotation at the time of starting and stopping the motor, an electromotive force of the motor is reduced, and thereby the surge pulse is reduced accordingly.
Also, in another sensorless method, a resistor is connected between certain two segments among multiple commutator segments of a commutator. In other words, the resistor is connected in parallel to a phase coil that is connected between the certain two segments. A rotation pulse is detected based on an electric current that flows between the two certain segments (see, for example, JP-A-2003-111465).
In the sensorless method described in JP-A-2003-111465, when the direct current is supplied through the brushes to a motor circuit, which, is a circuit of the armature coil having multiple phase coils, the electric current between the brushes periodically changes in accordance with the rotation angle of the motor because the resistor is connected in parallel to one of the phase coils. Because the rotation pulse is detected based on the change of the electric current, it is possible to improve a degree of detection accuracy compared with the detection method merely based on the surge pulse.
However, in the method described in JP-A-2003-111465, the rotation pulse is detected based on the periodical change of the direct current to the motor, which change is caused because the resistor is connected in parallel to the single phase coil. For example, when the direct current changes with other factors other than the connection to the resistor, such as the change of the power source voltage, the rotation pulse may be erroneously detected disadvantageously.